A memory card of an SD card or the like (hereinafter, simply referred to as a “memory card”), a USB memory, and the like are portable auxiliary storage devices adopting a flash memory as a recording medium. Since these are designed in a small size, electronic data may be optimally carried. For example, when a personal computer (PC) having a card slot into which the memory card can be inserted is possessed at both working place and home, it is possible to store and carry only required data in the memory card without carrying the PC from the home to the working place or from the working place to the home each time. This convenience receives a support of the markets and the number of sales is rapidly increasing.
Confidential data on business or the like is usually stored in the portable auxiliary storage device. On the other hand, it may be frequently lost due to smallness. When the portable auxiliary storage device falls into the hands of a malicious third person, there is a problem in that business data may be copied and distributed through the internet.
Recently, with the improvement of networking infrastructure, the opportunity to perform electronic commercial transaction (so-called e-commerce) through the internet is increasing. For example, a user conducts the electronic commercial transaction such as online stock trading even while traveling. The user conducts the electronic commercial transaction using a PC provided in a guest chamber of a hotel or the like or a PC permanently installed in a so-called internet cafe. Personal information (for example, a user ID or password, a bank account number, a telephone number, and the like) used for the transaction is stored and carried in the portable auxiliary storage device. When it is forgotten to extract the portable auxiliary storage device from the PC of the traveling place or the PC of the internet cafe, the third person may freely use the personal information.
As described above, the portable auxiliary storage device of the memory card or the like is convenient, but it is necessary to surely perform security protection and conventionally a countermeasure thereof is being variously considered.
For example, a technique has been disclosed for prohibiting the read of information from the memory card after a lapse of a given time by requesting a write to a dummy area of predetermined data when information is read from the memory card and providing a timer within the memory card to count the lapse of the given time from the dummy write with the timer (for example, see Patent Document 1).
[Patent Document 1] Japanese Patent Unexamined Publication No. 7-200767
A memory card described in the above-mentioned Patent Document 1 has a certain effect as an information leakage countermeasure. However, in the above-mentioned memory card, the read of information within the memory by a third person may be prohibited, but also the information read of a memory card possessor himself may be prohibited. The above-mentioned Patent Document 1 describes an indication to postpone the prohibition of the information read by reperforming a dummy write in a period shorter than a predetermined time before the lapse thereof in the timer count when the possessor knows an operation of this memory card.
However, when this dummy write is often requested to the user, the usability of the memory card is significantly damaged. This situation may be avoided by always preparing both a memory card having a data protection function and a general memory card (without a protection function), but the case where the user is requested to always prepare a plurality of memory cards is contrary to the convenience of the memory card.